Drugs, Trauma, and Stress, all affect Neurotransmitters. This interferes with normal communication in the brain.
Risk taking-including using drugs -causes dopamine systems to activate; this produces the feelings of extreme pleasure.
Dopamine Systems in the brain form the core of excitatory systems which include Oxytocin, and Norepinephrine. These neurotransmitters are natural evolutionary components necessary for learning and survival.
We do something healthy and the brain rewards us; therefore we will continue that act (e.g. procreation, eating, taking care of loved ones).
So, some people remember risky, or exciting experiences, because of these neural interactions. This encourages repetition of the act, while changing the structure of the neural networks.
Although the brain reaches 90% of its adult size by age six, the gray and white matter sub-components of the brain continue to undergo changes throughout adolescence.
The traditional explanation of adolescent behavior has been that it is due to the protracted development of the pre-frontal cortex. But we know it is much more than just that.
Adolescents use the Amygdala which is where emotions exist and nucleus accumbens (Limbic regions) rather than Prefrontal Cortex which provides reason and rationale.